An optical method for the rapid measurement of micromolar concentrations of nitrate in marine phytoplankton cultures

This method of nitrate determination by ultraviolet absorption spectrometry is based on the measurement of sample absorbance at a single wavelength (2 20 nm), which was chosen on the basis of the absorption spectra of the main components of artificial seawater in the ultraviolet domain. No reagents a re used and no sophisticated instruments are necessary. For standards prepa red in artificial seawater, the relationship between absorbance and nitrate concentration is linear up to 500 mu mol N L-1 and the detection limit is 1 mu mol N L-1. Precision is 1.5%. Urea and amino acids did not interfere a t concentrations typical of seawater. The method also measures nitrite, but this interference only becomes important for species which excrete large a mounts of nitrite. The method is extremely rapid, simple to implement and d oes not require the use of toxic chemicals such as cadmium. It should prove useful for monitoring quickly the nitrate concentrations in laboratory cul tures of marine phytoplankton.